1. Field of the Invention
The present invention relates generally to an apparatus and method for attenuating a leakage signal of a transmitter in a communication system. More particularly, the present invention relates to an apparatus and method for removing a leakage signal generated by an up-conversion mixer of a radio frequency (RF) transmitter, by branching a partial signal from the generated leakage signal, phase-shifting the branched partial signal, and combining the phase-shifted signal with an original signal.
2. Description of the Related Art
In a general communication system, an RF transmitter or receiver uses a direct conversion technique or a heterodyne technique. The direct conversion technique directly converts a low-frequency baseband signal and a high-frequency RF signal. The heterodyne technique converts a baseband signal or an RF signal into an intermediate frequency (IF) signal and then converts the IF signal into a signal of a desired frequency band. That is, unlike the direct conversion technique, the heterodyne technique performs an intermediate step of converting to an IF signal. Thus, compared to the direct conversion technique, the heterodyne technique can provide high signal sensitivity, selectivity and fidelity throughout a wide frequency band.
Therefore, a heterodyne technique is mainly used in most of the existing RF transmitters. The heterodyne technique uses an analog IF scheme or a digital IF scheme. The analog IF scheme converts a digital baseband signal into an analog signal using a digital-to-analog converter (DAC) and then converts the analog signal into a high-frequency RF signal by using an analog intermediate frequency. The digital IF scheme up-converts a digital baseband signal into an IF signal without converting the digital baseband signal into an analog signal. Unlike the analog IF scheme, the digital IF scheme can provide frequency conversion by digital control because it uses a digital signal.
As illustrated in FIG. 1, an RF transmitter using the heterodyne technique modulates an IF signal into an RF signal by using a mixer 101 to generate a frequency corresponding to a sum or difference of two frequencies received from a local oscillator (LO) 103 and a digital IF block 105. In an implementation, a signal isolation between ports of the mixer 101 may not have an infinite value. Thus, a leakage signal output from the LO 103 is input to the leakage signal input port as well as the RF and IF ports of the mixer 101, so that the mixer 101 generates a leakage signal 201 and an image signal 203 as well as a desired RF signal 205 as illustrated in FIG. 2. At this point, the image signal 203 is spaced apart from the leakage signal 201 by an intermediate frequency and has the same amplitude as the desired RF signal 205. The leakage signal 201 and the image signal 203 are input into a nonlinear device, which reduces the efficiency of the RF transmitter and generates an unnecessary component.
In order to remove the leakage signal 201 and the image signal 203, the RF transmitter uses an RF band-pass filter (BPF) 107 that passes only the desired RF signal 205 with a predetermined attenuation in the frequency band of the leakage signal 201 and the image signal 203 as illustrated in FIG. 2. However, as illustrated in FIG. 3A, when the RF transmitter uses a small IF signal, a leakage signal 301 and a desired RF signal 303 are near each other in terms of their frequency. In this case, the leakage signal 301 is insufficiently attenuated and the flatness characteristics in the vicinity of a pass-band boundary of the RF BPF may be degraded. In addition, as illustrated in FIG. 3B, when the RF transmitter uses a dual-band mode, a leakage signal 313 may exist between two desired RF signals 319 and 321. Thus, two or more RF BPFs must be used to remove two leakage signals 311 and 313 and two image signals 315 and 317 which increase the hardware implementation cost and the size of the RF transmitter.